1. Field of the Invention
This invention relates generally to magnetic material separators and is concerned more particularly with a magnetic drum separator provided with a permanent magnet structure for producing within the drum an axially varying magnetic field having a relatively high gradient as well as a relatively high intensity.
2. Discussion of the Prior Art
In the recycle processing of municipal trash materials, the "as received" items of raw trash generally are deposited in a conventional shredder wherein the items are reduced to particles of more manageable size. The resulting particles of commingled materials then may be directed into a stream, as by feeding them onto an endless conveyor belt, for example. The stream of particles from the shredder may be conducted to a conventional air classifier wherein light fraction materials, such as paper particles, for example, are removed from the stream. The particles of heavy fraction materials, such as metallic particles, for example, remaining in the stream may be conducted to a conventional magnetic separator for removal of ferromagnetic materials, such as iron and steel particles, for example. From the magnetic separator, the stream of nonferromagnetic materials may be conducted to a non-ferrous separator for removal of electrically conductive particles made of nonferromagnetic materials, such as aluminum, for example.
In U.S. Pat. No. 4,003,830 granted to E. Schloemann and assigned to the present assignee, there is shown a materials separator including an endless conveyor belt disposed for carrying a stream of heavy fraction particles around the periphery of a conventional magnetic drum separator. As a result, particles of ferromagnetic material in the stream are magnetically attracted and cling to the belt as it passes around the magnetic drum separator. When the belt carries the particles of ferromagnetic material beyond the influence of the magnetic drum separator, the particles of ferromagnetic material drop from the belt into a suitably disposed container. On the other hand, the particles of nonferromagnetic material in the stream leave the belt tangentially when the belt passes around the magnetic drum separator and are deposited in a chute which feeds the nonferromagnetic particles in a stream down a ramp-type non-ferrous materials separator.
The stream of nonferromagnetic particles slides down a smooth surface layer of nonmagnetic material overlying a permanent magnet array which induces eddy currents in the electrically conductive particles of nonferromagnetic material. These eddy currents produce magnetic fields which coact with the magnetic fields of the permanent magnet array to develop resultant forces having lateral components which deflect the electrically conductive particles of nonferromagnetic material laterally out of the stream. The particles of nonferromagnetic material which are not electrically conductive, such a glass, for example, are unaffected by the magnetic fields of the permanent magnet array and continue to slide linearly down the ramp-type separator.
However, in practice, it is found that the conventional magnetic materials separator may not remove all of the particles of ferromagnetic material from the stream. Composite particles comprising a small magnetic portion dominated by a larger non-magnetic portion may be prevented from clinging or even being attracted to the belt when it passes around the magnetic drum separator. These composite particles may include clothing having therein fasteners made of magnetic material, paper matchbooks having therein staples made of magnetic material, fasteners having therein eyelets made of magnetic material, and the like. Consequently, these composite particles of magnetic and nonmagnetic materials pass with the stream of nonferromagnetic particles to the ramptype nonferrous separator where they may be attracted sufficiently by the permanent magnet array to cling to the overlying smooth surface layer and clog the ramp-type separator thereby interferring with its efficient operation.